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I just bought a pair of MP60s used and one of them is humming loudly. It's the wetside for sure as it does it with both dry sides. I took it apart and it appears to be clean. I have linked to a video so the issue can be heard and hopefully solved. I reached out to the person i bought it from and they basically said tough luck.

Have you pulled the magnet out to make sure there isn't a snail under there or something like that?
Chipped impeller?
Seems unlikely, but I've seen the wet side cage miss the shaft on installation.
Spinning the square ceramic bearing (super glue it down if so)?
A good vinegar soak and a thin film of silicone grease if all else fails to make a difference, although at that point I'd be surprised if that did much.